Flame Sense Circuit for Gas Pilot Control

ABSTRACT

A flame sense circuit for a gas burner control employs a flame rod for flame rectification. An AC source applies an AC wave through a capacitor to the flame rod, and the flame rod is also connected via a resistive voltage divider to a DC voltage source. A voltage divider node is connected to a voltage follower, which impedance-matches an ADC input of a microprocessor controller. The flame sense circuit senses conditions of no-flame, and flame rod in contact with or leaking to ground, and when flame is present can quantify sensed flame as good, weak and marginal.

BACKGROUND OF THE INVENTION

This present invention relates to control devices for controlling gas fired appliances, such as furnaces and water heaters, and is more particularly concerned with a flame-sense arrangement that operates according to the principle of flame rectification.

The present invention is also concerned with a gas appliance control that employs an intermittent ignition system for the pilot light flame of the appliance in which the igniter or spark feature can also serve as a flame rod of the flame-sensor arrangement.

Flame rectification refers to a method of flame detection, using the property of a flame in which a plasma or ionized region within the flame serves as a unidirectional conductor, so that a current can flow from an interior of the flame towards the outside of the flame. Typically, a flame rod or conductive probe penetrates the flame envelope, and serves as anode, while a grounded pilot gas jet on the burner chassis serves as cathode. The flame then behaves as a diode connected in series with a large-value resistor, i.e., with a resistance value in the tens of megohms. The actual resistance value within the flame depends on the quality of the flame, and on the condition of the flame rod.

When an AC voltage is applied across the flame, e.g., using the thermostat power of 12 to 24 volts AC, the flame rectifies this, resulting in a net direct current on the order of about one micro-ampere (or smaller). This net direct current is then passed through a series of voltage divider resistances which have values on the order of several tens of megohms, and this develops a detectible voltage level. This can be used as an output voltage and applied to a later stage of the control. Any AC ripple is filtered out, i.e., diverted to ground, and a buffer amplifier can feed the voltage signal to a control input of another stage downstream. A voltage within a given range can be taken to mean that there is flame present in the burner. A voltage outside that range, i.e., at one or another higher levels, can mean that there is no flame present or can mean that there is leakage to ground from the flame rod. A voltage that is within one given voltage range or another can mean the presence of flame at the pilot gas burner, or can also mean that the flame is of marginal, poor, or good quality. A voltage that is outside that range, i.e., above it at a first or second high voltage level, can mean that flame is absent or that there is an electrical problem with the flame rod.

The controller can attempt to re-light the pilot flame, based on the output of the flame detector, or in some cases can shut off the gas valve and go into a lockout condition. Typically, the intermittent-operating igniter will apply spark voltage for a brief interval to attempt to ignite the pilot flame, and the flame sense circuit will sample flame quality during the interval between successive ignition intervals.

Up until now, these arrangements could only detect the presence or absence of flame, but have not been able to quantify or evaluate the condition of flame when it is present, even though that information would be useful during control installation or for troubleshooting. Previous flame rectification detectors have been unable to identify grounded flame rod conditions or conditions of leakage to ground, although that information also would be useful in installation of the control or in troubleshooting.

Examples of flame quality issues that would be useful, if they could be distinguished and detected, include a coated or contaminated flame rod; small pilot flame; or a waving pilot flame (i.e., not enveloping the flame rod), each of which can result in a weak flame current. Other installation or trouble shooting conditions include grounding due to mis-wiring during installation, ceramic insulator body breakdown, i.e., the insulator holding the igniter/flame rod becoming cracked or contaminated with the rod then coming into electrical contact with the chassis. Other quality issues on installation can occur if there is lead wire damage due to exposure to excessive temperatures, or contamination forming a conductive link to the chassis, i.e., build-up or coating on the ceramic body resulting in leakage current leakage from flame rod to ground.

Ideally, the flame sense arrangement should not only be able to detect presence/absence of flame, but should also be able to identify grounded flame rod conditions, and flame strength or quality conditions, and provide optical or other perceptible signals that correspond to the quality of the pilot light flame. There can be optical codes or indications for marginal flame, week flame, good flame, as well as whether flame is present. To date, this has not been achieved.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a flame rectification flame-sense circuit that overcomes the above-mentioned drawback(s) of the prior art, and that is capable of providing outputs corresponding to flame presence, flame absence, flame rod grounding to chassis, and flame strength or quality.

It is another object to provide a flame rectification flame sense circuit that achieves these objects using a simple and straightforward circuit arrangement, and which does not require a large number of components to do so.

In accordance with one aspect of the present invention, A flame sense circuit arrangement is provided for detecting and evaluating quality of a flame, e.g., the pilot burner flame, in a burner of a fuel-fired appliance, in which the gas burner has a chassis in electrical contact with a point of reference voltage, i.e., which can be considered “ground” or zero volts. A source of AC voltage is connected between such point of reference voltage (e.g., ground) and one electrode of a capacitor, the capacitor having a second electrode coupled to a first electrical node, i.e., node “A” in FIG. 1. A resistive voltage divider arrangement having a first resistance is connected between said first node “A” and a second node “B”. A second resistance is connected between the second node “B” and a source of DC voltage, e.g., +5 VDC. The flame rod has a conductor connected with the first node “A” and has one end thereof positioned to be in contact with the flame in the burner when flame is present. A voltage follower amplifier has a high impedance input coupled to the second node and is capable of receiving a net DC voltage component. The amplifier has a low impedance output to supply an output voltage level to the input of the processor arrangement from the low impedance output. The processor arrangement is capable of providing indications corresponding to respective flame conditions including flame absence, flame rod in contact with ground, and a plurality of flame quality levels. These can be different series of coded pulses, each of which corresponds to a flame condition, and the technician or installer can look those up in an installation guide. An indicator device (e.g., an LED or a cluster of LEDs) is or are coupled to the processor arrangement and can be adapted to provide perceptible indicia corresponding to said conditions of flame absence, flame rod in contact with ground, and the plurality of flame quality levels. A low-value protective resistor in series between the flame rod and the capacitor serves to prevent excessive currents in the event of a grounded flame rod. This can be a resistance of about 10 kilohms, and can be between the capacitor and the first node, or between the first node and the flame rod.

Favorably, the first resistance in the voltage divider has a value on the order of about 20 megohms to 50 megohms, and the second resistance has a value on the order of about 10 megohms. The capacitor may have a capacitance value on the order of about 2 nf.

Again, in a preferred mode the voltage follower amplifier is formed of an operational amplifier having a (+) input terminal coupled to the second node B, a (−) input terminal, and an output terminal that is coupled to the (−) input terminal and to an input of the processor arrangement. The voltage follower amplifier having an output impedance on the order of 5 to 10 Kilohms. A second capacitor may be coupled between the second node “B” and the point of reference voltage (e.g., ground) and is adapted to divert AC ripple from the (+) input terminal of the operational amplifier. The processor arrangement can also include an analog-to-digital converter input coupled to the output of the voltage follower amplifier, and is suitably programmed to provide respective signals to the indicator device depending on the voltage value at the output of the voltage follower amplifier.

In the control circuitry, a microprocessor is programmed to give a “flame-absent” indication when the voltage value is at a first predetermined voltage at or above a reference level (e.g., five volts), a “grounded flame rod” indication when the voltage value is at another predetermined voltage (e.g., 4 volts) below the first predetermined voltage, and one or more flame present indications when the voltage value is between the reference level (ground) and the second predetermined voltage (4 volts).

The microprocessor controller is programmed to provide a plurality of flame quality indications when said voltage value is within each of a plurality of respective zones between the second predetermined voltage and the reference level (e.g., ground).

In a favorable gas appliance arrangement, the flame rod is a pilot flame igniter electrode, and a spark coil secondary is connected in series with the flame rod and the first node “A”. A threshold breakdown neon switch device connected between said first node “A” and a chassis ground.

The indicator device includes one or more LEDs which provide a coded visual signal corresponding respectively to each of the flame conditions. These can be used also as an alarm to alert the homeowner to a service condition, so that repair or service can be obtained before a breakdown occurs.

The above and many other objects, features, and advantages of this invention will be more fully appreciated from the ensuing description of certain preferred embodiments, which are to be read in conjunction with the accompanying Drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a general schematic view of a flame sense circuit arrangement according to a preferred embodiment of this invention.

FIG. 2 is chart showing thr relation of output voltage to flame quality with this embodiment.

FIG. 3 is schematic view showing the flame sense circuit of this embodiment of this invention combined with spark or ignition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the Drawing, FIG. 1 shows a general arrangement of a flame sense circuit 10, for monitoring the quality of flame of a gas burner pilot 12. The pilot 12 has a chassis 14, which is electrically grounded with a flame 15, when present, extending upward, and with a flame rod 16, which is electrically isolated, extending into the envelope of the flame 15.

In the flame sense circuit 10, there is an AC voltage source 18, e.g., 12-volt or 24-volt AC 60 Hz thermostat power, coupled between a ground point and one terminal of a capacitor C1. The other terminal of the capacitor C1 is coupled to a junction point or node A, which is in turn connected to the flame rod 16. Favorably, this capacitor has a capacitance value of about 2.2 nanofarads, and there may be a protective resistor R1 between the capacitor C1 and the flame rod 16 to limit current in the event that the flame rod touches the chassis and grounds. The resistor R1 favorably has a resistance value of about 10 Kilohms. In this embodiment, the resistor R1 is situated between the capacitor C1 and the node A, but could alternatively be situated between the node A and the flame rod 16. The flame itself may act as a unidirectional conductor of low conductance, and is represented here in dash lines as a diode. When the alternating current is applied from the source 18 through the capacitor C1, the flame conducts a small current (on the order of one micro-ampere, or less) in the direction from the flame rod 16 to the grounded chassis 14. This results in a net negative voltage accumulating onto the upper plate of the capacitor C1, as illustrated. The net dc voltage level at the capacitor C1 depends on the position, size and quality of the flame 15.

A resistive voltage divider 20 extends from the node A to a source of DC (“battery”) voltage Vdd, typically about +5 volts. The divider 20 includes a pair of resistors R2 and R3 connected in series between node A and a second node B, and a third resistor R4 connected between the node B and the DC voltage source Vdd. The resistors R2 and R3 are each in the megohm range, i.e., each 10 MΩ to 22 MΩ and resistor R4 preferably has a value of about 10 MΩ.

A second capacitor C2 is coupled between the second node B and a reference voltage to shunt AC ripple. The node B2 is also connected to a voltage follower circuit, here formed of an operation amplifier 22 or OpAmp. A net DC level then appears on the (+) input of the OpAmp 22. The OpAmp is configured with the output terminal coupled to the (−) input, and there is an output resistor R5 at the output terminal. This resistor R5 has a resistance value of about 4.7 to 10 KΩ, and is selected to match the input impedance of an input of an analog-to-digital functionality 24 of a microprocessor controller. This stage then converts the analog voltage from the OpAmp to digital form and supplies it to a controller stage 26 in the microprocessor controller. This stage interprets the DC level that appears node B and at the OpAmp output, in accordance with a program or algorithm based on the voltage chart shown in FIG. 2, explained just below. The microprocessor controller then sends a flash code to a visible indicator LED 28 to indicate the flame status, i.e., flame absent, flame marginal, flame weak, flame good, or flame rod grounded. The code can be a number of flashes in a group, e.g., with a single flash indicating good flame, two flashes meaning weak flame, three meaning marginal flame, no flashes meaning no flame detected, and a solid ON condition meaning flame rod grounded. Other flash codes for these and other conditions are possible, using e.g., combinations of short and long flashes.

The power supply for the OpAmp 22 may be regulated to 6.5 V DC which enables a complete output range of 0 VDC to 5 Vdc at the (+) input to the ADC functionality, and can provide better resolution between flame condition zones, as explained below in reference to FIG. 2.

It can be seen that when flame is absent, or not in contact with the flame rod, then no net current flows through the flame rod, and the capacitor is not pulled negative. Under that condition, the voltage waveform at point A is a superposition of an AC signal and a small positive DC component from the voltage divider 10. The voltage at the node B is pulled up to battery voltage Vdd, e.g., +5 volts, and that voltage also appears at the input of the ADC functionality 24. This is in the no-flame zone of FIG. 2. Resistive conduction between the flame rod and chassis ground will not change the symmetry between positive and negative half cycles of the AC waveform but only reduces its amplitude. This results in an output voltage being in a Resistive Conduction zone that is shared with the No Flame zone, as shown in FIG. 2. If there is flame rod contact with the chassis, then node A appears is at ground, and the voltage at the node B provides the divided voltage: V_(out)=Vdd×(R2+R3)/(R2+R3+R4), which in this example would be about +4 volts. A narrow zone appears here to allow for resistance of the flame rod being non-zero, and allowing for voltage ripple. The flame threshold would be at a voltage below that level. Where flame is present, and flame is in contact with the flame rod, the rectification effect of the flame will pull the net voltage of the capacitor C1 negative. The effect is smaller for marginal and weak flame conditions, and larger for normal flame conditions, so there are “marginal”, “weak” and “good” flame conditions defined for voltage ranges as indicated in FIG. 2. The voltage output will be in a Flame zone, which can be resolved further into sub zones: Good, Weak, Marginal. A Flame-Below-Threshold zone is also shown in FIG. 2, and is used as a stability or buffer zone, such as when sensed flame is too low and is considered to be below threshold, with the system thus being unstable. This is considered the same as No Flame, for control purposes.

The flame sensing circuit 10 can also be used for sensing flame through the spark transformer or ignition coil as shown in FIG. 3, where the control circuit is an intermittent pilot gas ignition control. Here spark is applied intermittently, and flame is sensed during the spark pause interval. Here the gas pilot 12 is shown with the flame rod 16 mounted on the chassis 14, with a tip of the flame rod facing a grounded spark electrode 30. A ceramic insulator body 32 mounts the flame rod 16 to the chassis 14. A spark transformer or ignition coil secondary 34 is coupled to the flame rod 16 with also serves as the high voltage spark electrode. A switching spark gap 36 connects at a node C to the other end of the secondary 34, and conducts to ground if the voltage V on the node C exceeds some threshold, e.g., 90 volts. The flame sense circuit 10 is also joined to the flame rod 16 (through the ignition coil secondary 34) at node C, and the spark gap 36 diverts spark voltage away from the flame sense circuit. The spark transformer or ignition coil 34 can be energized intermittently, e.g., at one second intervals, and the flame sense operation can be carried out during the pause intervals between ignition attempts.

The indicator LED 28 can provide flash codes associated with the respective flame zones shown in FIG. 2. Pulses or flashes at a steady heart-beat rate can mean Good Flame, two flashes and a pause can mean Weak, one flash and a pause can mean Marginal, and no flashes can mean no flame, or flame below threshold. A steady ON can mean grounded flame rod. The circuit can (with only minor modification) be used to measure and indicate or display flame current. This can be the equivalent of indicating or displaying the flame current that a micro-ammeter shows when in series with the flame rod.

While the invention has been described with reference to specific preferred embodiments, the invention is certainly not limited to those precise embodiments. Rather, many modifications and variations will become apparent to persons of skill in the art without departure from the scope and spirit of this invention, as defined in the appended claims. 

1. A flame sense circuit arrangement for detecting and quantifying a sensed flame in a burner of a fuel-fired appliance, comprising: a gas burner having a chassis in electrical contact with a point of reference voltage; a source of AC voltage connected between said point of reference voltage and one electrode of a capacitor, the capacitor having a second electrode coupled to a first electrical node A; a resistive voltage divider arrangement having a first resistance connected between said first node A and a second node B, and a second resistance connected between said second node B and a source of DC voltage; a flame rod having a conductor connected with said first node A and having one end thereof positioned to be in contact with the flame in said burner when flame is present; a voltage follower amplifier having a high impedance input coupled to said second node B and capable of receiving a net DC voltage component, and having a low impedance output; a processor arrangement coupled to said low impedance output and capable of providing indications corresponding to respective flame conditions including flame absence, flame rod in contact with ground, and a plurality of sensed flame quantity levels; and an indicator device coupled to said processor arrangement and adapted to provide perceptible indicia corresponding to said conditions of flame absence, flame rod in contact with ground, and said plurality of flame quality levels.
 2. The flame sense circuit arrangement according to claim 1, comprising a low-value protective resistor in series between said flame rod and said capacitor.
 3. The flame sense circuit arrangement according to claim 1, wherein said first resistance has a value on the order of about 20 megohms to 50 megohms, and said second resistance has a value on the order of about 10 megohms.
 4. The flame sense circuit arrangement according to claim 1, wherein said capacitor has a capacitance value on the order of about 2 nf.
 5. The flame sense circuit arrangement according to claim 1, wherein said voltage follower amplifier is formed of an operational amplifier having a (+) input terminal coupled to said second node B, a (−) input terminal, and an output terminal that is coupled to said (−) input terminal and to an input of said processor arrangement.
 6. The flame sense circuit arrangement according to claim 5, said voltage follower amplifier having an output impedance on the order of 10 Kilohms.
 7. The flame sense circuit arrangement according to claim 5, comprising a second capacitor coupled between the second node B and said point of reference voltage and adapted to filter net DC voltage to the (+) input terminal of said operational amplifier.
 8. The flame sense circuit arrangement according to claim 1, wherein said processor arrangement includes an analog-to-digital converter having an input coupled to said indicator device.
 9. The flame sense circuit arrangement according to claim 8, wherein said microprocessor is programmed to give a flame-absent indication when the voltage value is at a first predetermined voltage above a reference level, a grounded flame rod indication when said voltage value is at another predetermined voltage below said first predetermined voltage, and to provide one or more flame present indications when said voltage value is between said reference level and said second predetermined voltage.
 10. The flame sense circuit arrangement according to claim 9, wherein said microprocessor is programmed to provide a plurality of sensed flame quantity indications when said voltage value is within each of a plurality of zones between said second predetermined voltage and said reference level.
 11. The flame sense circuit arrangement according to claim 1, wherein said flame rod is a pilot flame igniter electrode, and said circuit arrangement further includes a spark coil secondary connected in series with said flame rod and said first node A, and a threshold breakdown switch device connected between said first node A and a chassis ground.
 12. The flame sense circuit arrangement according to claim 10, wherein said indicator device includes one or more LEDs which provide coded visual signals corresponding respectively to each of said conditions. 